Separate Universe Simulations Research Articles

PNG-UNITsims: Halo clustering response to primordial non-Gaussianities as a function of mass

This paper presents the PNG-UNITSIMS suite, which includes the largest full N-body simulation to date with local primordial non-Gaussianities (local PNG), the PNG-UNIT. The amplitude of the PNGs is given by f localNL=100. The simulation follows the evolution of 40963 particles in a periodic box with Lbox = 1 h−1 Gpc, resulting in a mass resolution of mp = 1.24 × 109 h−1 M⊙, enough to finely resolve the galaxies targeted by stage-IV spectroscopic surveys. The PNG-UNIT has fixed initial conditions with phases also matching the pre-existing UNIT simulation with Gaussian initial conditions. The fixed and matched initial conditions reduce the simulation uncertainty significantly. In this first study of the PNG-UNITSIMS, we measure the PNG response parameter, p, as a function of the halo mass. halos with masses between 1 × 1012 and 5 × 1013 h−1 M⊙ are well described by the universality relation, given by p = 1. For halos with masses between 2 × 1010 and 1 × 1012 h−1 M⊙ we find that p < 1, at a significance between 1.5 and 3.1σ. Combining all the halos between 2 × 1010 and 5 × 1013 h−1 M⊙, we find p consistent with a value of 0.955 ± 0.013, which is 3σ away from the universality relation. We demonstrate that these findings are robust to mass resolution, scale cuts and uncertainty estimation. We also compare our measurements to separate universe simulations, finding that the PNG-UNITSIMS constraints outperform the former for the setup considered. Using a prior on p as tight as the one reported here for DESI-like forecast can result in fNL constraints comparable to fixing p. At the same time, fixing p to a wrong value (p = 1) may result in up to 2σ biases on fNL.

Constraints on compensated isocurvature perturbations from BOSS DR12 galaxy data

We use the BOSS DR12 galaxy power spectrum to constrain compensated isocurvature perturbations (CIP), which are opposite-sign primordial baryon and dark matter perturbations that leave the total matter density unchanged. Long-wavelength CIP σ(x) enter the galaxy density contrast as δg (x) ⊃ bσσ(x), with bσ the linear CIP galaxy bias parameter. We parameterize the CIP spectra as Pσσ = A 2 Pℛℛ and Pσℛ = ξ√PσσPℛℛ , where A is the CIP amplitude and ξ is the correlation with the curvature perturbations ℛ. We find a significance of detection of Ab σ ≠ 0 of 1.8σ for correlated CIP (ξ = 1), consistent with no detection. For uncorrelated CIP (ξ = 0), the constraints are instead more significantly shifted away from zero, although this may be due to large-scale data systematics which have a bigger impact on these type of CIP. The constraints on A depend on the assumed priors for the bσ parameter, which we estimate using separate universe simulations. Assuming bσ values representative of all halos we find σA = 145 for correlated CIP and σ |A| = 475 for uncorrelated CIP. Our strongest uncorrelated CIP constraint is for bσ representative of the 33% most concentrated halos, σ |A| = 197, which is better than the current CMB bounds |A| ≲ 360. We also discuss the impact of the local primordial non-Gaussianity parameter f NL in CIP constraints. Our results demonstrate the power of galaxy data to place tight constraints on CIP, and motivate works to understand better the impact of data systematics, as well as to determine theory priors for bσ .

Gravitational wave fossils in nonlinear regime: Halo tidal bias and intrinsic alignments from gravitational wave separate universe simulations

We investigate impacts of long-wavelength gravitational waves (GWs) on nonlinear structure formation by utilizing the tidal separate universe simulations. Based on the equivalence of a long-wavelength GW to a uniform tidal field in a local frame, we provide a way to incorporate a long-wavelength GW into the tidal separate universe simulation as an effective anisotropic expansion. This methodology enables us to study effects of GWs on large-scale structure efficiently. We measure the anisotropic imprint in the local power spectrum from the tidal separate universe simulations with GWs, which corresponds to the scalar-scalar-tensor bispectrum in squeezed limit or the so-called power spectrum response to GWs. We also detect the halo tidal bias induced by GWs from the response of the halo-matter cross-power spectrum to GWs, as well as the linear shape bias (or the linear alignment coefficient) induced by GWs from the one-point function of the halo ellipticity. In contrast to the case of the tidal field induced by scalar perturbations, we discover that the wavenumber dependence of the temporal evolution of GWs naturally causes these biases to be scale-dependent. We also find that this scale dependence is well approximated by the second-order density induced by the coupling between scalar and tensor perturbation. This highlights that the structure formation, especially the process to determine the halo shape, is nonlocal in time. Our findings lay the foundation for predicting the impact of GWs on large-scale structure.

The local PNG bias of neutral Hydrogen, HI

We use separate universe simulations with the IllustrisTNG galaxy formation model to predict the local PNG bias parameters bΦ and bΦδ of atomic neutral hydrogen, HI. These parameters and their relation to the linear density bias parameter b 1 play a key role in observational constraints of the local PNG parameter f NL using the HI power spectrum and bispectrum. Our results show that the popular calculation based on the universality of the halo mass function overpredicts the bΦ (b 1) and bΦδ (b 1) relations measured in the simulations. In particular, our results show that at z ≲ 1 the HI power spectrum is more sensitive to f NL compared to previously thought (bΦ is more negative), but is less sensitive at other epochs (bΦ is less positive). We discuss how this can be explained by the competition of physical effects such as that large-scale gravitational potentials with local PNG (i) accelerate the conversion of hydrogen to heavy elements by star formation, (ii) enhance the effects of baryonic feedback that eject the gas to regions more exposed to ionizing radiation, and (iii) promote the formation of denser structures that shield the HI more efficiently. Our numerical results can be used to revise existing forecast studies on f NL using 21 cm line-intensity mapping data. Despite this first step towards predictions for the local PNG bias parameters of HI, we emphasize that more work is needed to assess their sensitivity on the assumed galaxy formation physics and HI modeling strategy.

Predictions for local PNG bias in the galaxy power spectrum and bispectrum and the consequences for f NL constraints

We use hydrodynamical separate universe simulations with the IllustrisTNG model to predict the local primordial non-Gaussianity (PNG) bias parameters b ϕ and b ϕδ, which enter at leading order in the galaxy power spectrum and bispectrum. This is the first time that b ϕδ is measured from either gravity-only or galaxy formation simulations. For dark matter halos, the popular assumption of universality overpredicts the b ϕδ(b 1) relation in the range 1 ≲ b 1 ≲ 3 by up to Δ b ϕδ ∼ 3 (b 1 is the linear density bias). The adequacy of the universality relation is worse for the simulated galaxies, with the relations b ϕ(b 1) and b ϕδ(b 1) being generically redshift-dependent and very sensitive to how galaxies are selected (we test total, stellar and black hole mass, black hole mass accretion rate and color). The uncertainties on b ϕ and b ϕδ have a direct, often overlooked impact on the constraints of the local PNG parameter f NL, which we study and discuss. For a survey with V = 100 Gpc3/h3 at z=1, uncertainties Δ b ϕ ≲ 1 and Δ b ϕδ ≲ 5 around values close to the fiducial can yield relatively unbiased constraints on f NL using power spectrum and bispectrum data. We also show why priors on galaxy bias are useful even in analyses that fit for products f NL b ϕ and f NL b ϕδ. The strategies we discuss to deal with galaxy bias uncertainties can be straightforwardly implemented in existing f NL constraint analyses (we provide fits for some of the bias relations). Our results motivate more works with galaxy formation simulations to refine our understanding of b ϕ and b ϕδ towards improved constraints on f NL.

Position-dependent Voronoi probability distribution functions for matter and halos

We measure the Voronoi density probability distribution function (PDF) for both dark matter and halos in N-body simulations. For the dark matter, Voronoi densities represent the matter density field smoothed on a uniform mass scale, which approximates the Lagrangian density field. For halos, the Voronoi densities contain information about the local environment of each halo. We measure the halo virial masses, the total amount of dark matter within each halo Voronoi cell, and the halo Voronoi cell volumes, and we show how halo abundances depend on these three quantities. We then study the position-dependent Voronoi density PDF, measured within finite subregions of the Universe, using separate universe simulations. We demonstrate that the spatial variation of the position-dependent PDF is due to large-scale density fluctuations, indicating that the position-dependent PDF is a biased tracer of large-scale structure. We measure this bias for the dark matter, and interpret it as the bias of regions of the Lagrangian density field that are selected based on density. For the halos, this bias can be interpreted as a form of assembly bias. We present the mapping from late-time to early-time Voronoi density for each simulation dark matter particle, which is highly stochastic. We compare the median of this stochastic map with spherical collapse calculations and discuss challenges involved in modeling the evolution of the density field on these scales.

Responses of Halo Occupation Distributions: a new ingredient in the halo model & the impact on galaxy bias

Halo occupation distribution (HOD) models describe the number of galaxies that reside in different haloes, and are widely used in galaxy-halo connection studies using the halo model (HM). Here, we introduce and study HOD response functions R \U0001d4aa g that describe the response of the HODs to long-wavelength perturbations \U0001d4aa. The linear galaxy bias parameters b \U0001d4aa g are a weighted version of b \U0001d4aa h + R \U0001d4aa g , where b \U0001d4aa h is the halo bias, but the contribution from R \U0001d4aa g is routinely ignored in the literature. We investigate the impact of this by measuring the R \U0001d4aa g in separate universe simulations of the IllustrisTNG model for three types of perturbations: total matter perturbations, \U0001d4aa = δ h ; baryon-CDM compensated isocurvature perturbations, \U0001d4aa = σ; and potential perturbations with local primordial non-Gaussianity, \U0001d4aa ∝ f NLϕ. Our main takeaway message is that the R \U0001d4aa g are not negligible in general and their size should be estimated on a case-by-case basis. For stellar-mass selected galaxies, the responses R \U0001d4aa g and R σ g are sizeable and cannot be neglected in HM calculations of the bias parameters b ϕ g and b σ g ; this is relevant to constrain inflation using galaxies. On the other hand, we do not detect a strong impact of the HOD response R 1 g on the linear galaxy bias b 1 g . These results can be explained by the impact that the perturbations \U0001d4aa have on stellar-to-total-mass relations. We also look into the impact on the bias of the gas distribution and find similar conclusions. We show that a single extra parameter describing the overall amplitude of R \U0001d4aa g recovers the measured b \U0001d4aa g well, which indicates that R \U0001d4aa g can be easily added to HM/HOD studies as a new ingredient.

Galaxy bias and primordial non-Gaussianity: insights from galaxy formation simulations with IllustrisTNG

We study the impact that large-scale perturbations of (i) the matter density and (ii) the primordial gravitational potential with local primordial non-Gaussianity (PNG) have on galaxy formation using the IllustrisTNG model. We focus on the linear galaxy bias b1 and the coefficient bϕ of the scale-dependent bias induced by PNG, which describe the response of galaxy number counts to these two types of perturbations, respectively. We perform our study using separate universe simulations, in which the effect of the perturbations is mimicked by changes to the cosmological parameters: modified cosmic matter density for b1 and modified amplitude \U0001d49cs of the primordial scalar power spectrum for bϕ. We find that the widely used universality relation bϕ = 2δc(b1 − 1) is a poor description of the bias of haloes and galaxies selected by stellar mass M*, which is instead described better by bϕ(M*) = 2δc(b1(M*) − p) with p ∊ [0.4, 0.7]. This is explained by the different impact that matter overdensities and local PNG have on the median stellar-to-halo-mass relation. A simple model of this impact allows us to describe the stellar mass dependence of b1 and bϕ fairly well. Our results also show a nontrivial relation between b1 and bϕ for galaxies selected by color and black hole mass accretion rate. Our results provide refined priors on bϕ for local PNG constraints and forecasts using galaxy clustering. Given that the widely used universality relation underpredicts bϕ(M*), existing analyses may underestimate the true constraining power on local PNG.

Impacts of pre-initial conditions on anisotropic separate universe simulations: a boosted tidal response in the epoch of reionization

ABSTRACT To generate initial conditions for cosmological N-body simulations, one needs to prepare a uniform distribution of simulation particles, the so-called pre-initial condition (pre-IC). The standard method to construct the pre-IC is to place the particles on the lattice grids evenly spaced in the three-dimensional spatial coordinates. However, even after the initial displacement of each particle according to cosmological perturbations, the particle distribution remains to display an artificial anisotropy. Such an artefact causes systematic effects in simulations at later time until the evolved particle distribution sufficiently erases the initial anisotropy. In this paper, we study the impacts of the pre-IC on the anisotropic separate universe simulation, where the effect of large-scale tidal field on structure formation is taken into account using the anisotropic expansion in a local background (simulation volume). To quantify the impacts, we compare the simulations employing the standard grid pre-IC and the glass one, where the latter is supposed to suppress the initial anisotropy. We show that the artificial features in the grid pre-IC simulations are seen until z ∼ 9, while the glass pre-IC simulations appear to be stable and accurate over the range of scales we study. From these results we find that a coupling of the large-scale tidal field with matter clustering is enhanced compared to the leading-order prediction of perturbation theory in the quasi-non-linear regime in the redshift range 5 ≲ z ≲ 15, indicating the importance of tidal field on structure formation at such high redshifts, e.g. during the epoch of reionization.

Effective halo model: Creating a physical and accurate model of the matter power spectrum and cluster counts

We introduce a physically motivated model of the matter power spectrum, based on the halo model and perturbation theory. This model achieves 1% accuracy on all $k$-scales between $k=0.02h\text{ }\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}$ to $k=1h\text{ }\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}$. Our key ansatz is that the number density of halos depends on the nonlinear density contrast filtered on some unknown scale $R$. Using the effective field theory of large scale structure to evaluate the two-halo term, we obtain a model for the power spectrum with only two fitting parameters: $R$ and the effective ``sound speed,'' which encapsulates small-scale physics. This is tested with two suites of cosmological simulations across a broad range of cosmologies and found to be highly accurate. Due to its physical motivation, the statistics can be easily extended beyond the power spectrum; we additionally derive the one-loop covariance matrices of cluster counts and their combination with the matter power spectrum. This yields a significantly better fit to simulations than previous models, and includes a new model for supersample effects, which is rigorously tested with separate universe simulations. At low redshift, we find a significant ($\ensuremath{\sim}10%$) exclusion covariance from accounting for the finite size of halos which has not previously been modeled. Such power spectrum and covariance models will enable joint analysis of upcoming large-scale structure surveys, gravitational lensing surveys, and cosmic microwave background maps on scales down to the nonlinear scale. We provide a publicly released python code.

Measurement of Void Bias Using Separate Universe Simulations

Abstract Cosmic voids are biased tracers of the large-scale structure of the universe. Separate universe simulations (SUS) enable accurate measurements of this biasing relation by implementing the peak-background split (PBS). In this work, we apply the SUS technique to measure the void bias parameters. We confirm that the PBS argument works well for underdense tracers. The response of the void size distribution depends on the void radius. For voids larger (smaller) than the size at the peak of the distribution, the void abundance responds negatively (positively) to a long wavelength mode. The linear bias from the SUS is in good agreement with the cross power spectrum measurement on large scales. Using the SUS, we have detected the quadratic void bias for the first time in simulations. We find that b 2 is negative when the magnitude of b 1 is small, and that it becomes positive and increases rapidly when increases. We compare the results from voids identified in the halo density field with those from the dark matter distribution, and find that the results are qualitatively similar, but the biases generally shift to the larger voids sizes.

Separate universe void bias

Voids have emerged as a novel probe of cosmology and large-scale structure. These regions of extreme underdensity are sensitive to physics beyond the standard model of cosmology, and can potentially be used as a testing ground to constrain new physics. We present the first determination of the linear void bias measured in separate universe simulations. Our methods are validated by comparing the separate universe response bias with the clustering bias of voids. We find excellent agreement between the two methods for voids identified in the halo field and the down-sampled dark matter field. For voids traced by halos, we identify two different contributions to the bias. The first is due to the bias of the underlying halo field used to identify voids, while the second we attribute to the dynamical impact of long-wavelength density perturbations on void formation and expansion. By measuring these contributions individually, we demonstrate that their sum is consistent with the total void bias. We also measure the void profiles in our simulations, and determine their separate universe response. These can be interpreted as the sensitivity of the profiles to the background density of the Universe.

Quintessential isocurvature in separate universe simulations

In a universe with quintessence isocurvature, or perturbations in dark energy that are independent from the usual curvature perturbations, structure formation is changed qualitatively. The existence of two independent fields, curvature and isocurvature, causes the growth rate of matter perturbations to depend on their initial conditions. The quintessence perturbations cause their growth to depend on scale. We perform the first separate universe simulations for this cosmology. We demonstrate that the power spectrum response and the halo bias depend on scale and initial conditions and that the presence of the isocurvature mode changes the mapping from these quantities to the halo auto- and cross-power spectra, and the squeezed-limit bispectrum. We compare the bias to several models, finding reasonable agreement with both a power-spectrum-response model with one free parameter and a model that fits two independent bias parameters for curvature and isocurvature sourced fluctuations. We also verify that simulation responses to pure isocurvature and pure curvature modes can be linearly combined to reproduce responses with different ratios of isocurvature and curvature. This allows our results to be used to predict the halo power spectrum and stochasticity with arbitrary large-scale curvature and isocurvature power spectra. In an appendix, we study the generation of quintessence isocurvature during inflation and show that a modified kinetic term is typically required to produce observable isocurvature modes in a field with $w_Q\approx -1$.

Separate Universe simulations with IllustrisTNG: baryonic effects on power spectrum responses and higher-order statistics

Abstract We measure power spectrum response functions in the presence of baryonic physical processes using separate universe simulations with the IllustrisTNG galaxy formation model. The response functions describe how the small-scale power spectrum reacts to long-wavelength perturbations and they can be efficiently measured with the separate universe technique by absorbing the effects of the long modes into a modified cosmology. Specifically, we focus on the total first-order matter power spectrum response to an isotropic density fluctuation R1(k, z), which is fully determined by the logarithmic derivative of the non-linear matter power spectrum dlnPm(k, z)/dlnk and the growth-only response function G1(k, z). We find that G1(k, z) is not affected by the baryonic physical processes in the simulations at redshifts z < 3 and on all scales probed (k ≲ 15 h Mpc−1; i.e. length scales $\gtrsim 0.4\, {\rm Mpc}\,h^{-1}$). In practice, this implies that the power spectrum fully specifies the baryonic dependence of its response function. Assuming an idealized lensing survey set-up, we evaluate numerically the baryonic impact on the squeezed-lensing bispectrum and the lensing supersample power spectrum covariance, which are given in terms of responses. Our results show that these higher-order lensing statistics can display varying levels of sensitivity to baryonic effects compared to the power spectrum, with the squeezed bispectrum being the least sensitive. We also show that ignoring baryonic effects on lensing covariances slightly overestimates the error budget (and is therefore conservative from the point of view of parameter error bars) and likely has negligible impact on parameter biases in inference analyses.

First Detection of Scale-Dependent Linear Halo Bias in N-Body Simulations with Massive Neutrinos.

Using N-body simulations with massive neutrino density perturbations, we detect the scale-dependent linear halo bias with high significance. This is the first time that this effect is detected in simulations containing neutrino density perturbations on all scales, confirming the same finding from separate universe simulations. The scale dependence is the result of the additional scale in the system, i.e., the massive neutrino free-streaming length, and it persists even if the bias is defined with respect to the cold dark matter plus baryon (instead of total matter) power spectrum. The separate universe approach provides a good model for the scale-dependent linear bias, and the effect is approximately 0.25f_{ν} and 0.43f_{ν} for halos with bias of 1.7 and 3.5, respectively. While the size of the effect is small, it is not insignificant in terms of f_{ν} and should therefore be included to accurately constrain neutrino mass from clustering statistics of biased tracers. More importantly, this feature is a distinct signature of free-streaming particles and cannot be mimicked by other components of the standard cosmological model.

Covariances for cosmic shear and galaxy–galaxy lensing in the response approach

In this study, we measure the response of matter and halo projected power spectra $P^{\rm 2D}_{\rm XY}(k)$ (X, Y are matter and/or halos), to a large-scale density contrast, $\delta_{\rm b}$, using separate universe simulations. We show that the fractional response functions, i.e., $\mathrm{d}\ln P^{\rm 2D}_{\rm XY}(k)/\mathrm{d}\delta_{\rm b}$, are identical to their respective three-dimensional power spectra within simulation measurement errors. Then, using various $N$-body simulation combinations (small-box simulations with periodic boundary conditions and sub-volumes of large-box simulations) to construct {mock observations of projected fields}, we study how super-survey modes, in both parallel and perpendicular directions to the projection direction, affect the covariance matrix of $P^{\rm 2D}_{\rm XY}(k)$, known as super-sample covariance (SSC). Our results indicate that the SSC term provides dominant contributions to the covariances of matter-matter and matter-halo spectra at small scales but does not provide significant contributions in the halo-halo spectrum. We observe that the large-scale density contrast in each redshift shell causes most of the SSC effect, and we did not observe a SSC signature arising from large-scale tidal field within the levels of measurement accuracy. We also develop a response approach to calibrate the SSC term for cosmic shear correlation function and galaxy--galaxy weak lensing, and validate the method by comparison with the light-cone ray-tracing simulations. Our method provides a reasonably accurate, albeit computationally inexpensive, way to calibrate the covariance matrix for clustering observables available from wide-area galaxy surveys.

Scale-dependent bias and bispectrum in neutrino separate universe simulations

Cosmic background neutrinos have a large velocity dispersion, which causes the evolution of long-wavelength density perturbations to depend on scale. This scale-dependent growth leads to the well-known suppression in the linear theory matter power spectrum that is used to probe neutrino mass. In this paper, we study the impact of long-wavelength density perturbations on small-scale structure formation. By performing separate universe simulations where the long-wavelength mode is absorbed into the local expansion, we measure the responses of the cold dark matter (CDM) power spectrum and halo mass function, which correspond to the squeezed-limit bispectrum and halo bias. We find that the scale-dependent evolution of the long-wavelength modes causes these quantities to depend on scale and provide simple expressions to model them in terms of scale and the amount of massive neutrinos. Importantly, this scale-dependent bias reduces the suppression in the linear halo power spectrum due to massive neutrinos by 13 and 26% for objects of bias $\bar{b}=2$ and $\bar{b} \gg1$, respectively. We demonstrate with high statistical significance that the scale-dependent halo bias ${\it cannot}$ be modeled by the CDM and neutrino density transfer functions at the time when the halos are identified. This reinforces the importance of the temporal nonlocality of structure formation, especially when the growth is scale dependent.

Complete super-sample lensing covariance in the response approach

We derive the complete super-sample covariance (SSC) of the matter and weak lensing convergence power spectra using the power spectrum response formalism to accurately describe the coupling of super- to sub-survey modes. The SSC term is completely characterized by the survey window function, the nonlinear matter power spectrum and the full first-order nonlinear power spectrum response function, which describes the response to super-survey density and tidal field perturbations. Generalized separate universe simulations can efficiently measure these responses in the nonlinear regime of structure formation, which is necessary for lensing applications. We derive the lensing SSC formulae for two cases: one under the Limber and flat-sky approximations, and a more general one that goes beyond the Limber approximation in the super-survey mode and is valid for curved sky applications. Quantitatively, we find that for sky fractions fsky ≈ 0.3 and a single source redshift at zS=1, the use of the flat-sky and Limber approximation underestimates the total SSC contribution by ≈ 10%. The contribution from super-survey tidal fields to the lensing SSC, which has not been included in cosmological analyses so far, is shown to represent about 5% of the total lensing covariance on multipoles ℓ1,ℓ2 ≳ 300. The SSC is the dominant off-diagonal contribution to the total lensing covariance, making it appropriate to include these tidal terms and beyond flat-sky/Limber corrections in cosmic shear analyses.

Halo squeezed-limit bispectrum with primordial non-Gaussianity: A power spectrum response approach

Modeling the nonlinearity of the halo bispectrum remains a major challenge in modern cosmology, in particular for ongoing and upcoming large-scale structure observations that are performed to study the inflationary physics. The "power spectrum response" offers a solution for bispectrum in the so-called squeezed limit, in which one wavenumber is much smaller than the other two. As a first step, we demonstrate that the halo squeezed-limit bispectrum computed from the second-order standard perturbation theory agrees precisely with the responses of linear halo power spectrum to large-scale density and potential fluctuations. Since the halo power spectrum responses to arbitrarily small scales can straightforwardly be obtained by separate universe simulations, the response approach provides a novel and powerful technique for modeling the nonlinear halo squeezed-limit bispectrum.

Response approach to the squeezed-limit bispectrum: application to the correlation of quasar and Lyman-α forest power spectrum

The squeezed-limit bispectrum, which is generated by nonlinear gravitational evolution as well as inflationary physics, measures the correlation of three wavenumbers, in the configuration where one wavenumber is much smaller than the other two. Since the squeezed-limit bispectrum encodes the impact of a large-scale fluctuation on the small-scale power spectrum, it can be understood as how the small-scale power spectrum ``responds'' to the large-scale fluctuation. Viewed in this way, the squeezed-limit bispectrum can be calculated using the response approach even in the cases which do not submit to perturbative treatment. To illustrate this point, we apply this approach to the cross-correlation between the large-scale quasar density field and small-scale Lyman-α forest flux power spectrum. In particular, using separate universe simulations which implement changes in the large-scale density, velocity gradient, and primordial power spectrum amplitude, we measure how the Lyman-α forest flux power spectrum responds to the local, long-wavelength quasar overdensity, and equivalently their squeezed-limit bispectrum. We perform a Fisher forecast for the ability of future experiments to constrain local non-Gaussianity using the bispectrum of quasars and the Lyman-α forest. Combining with quasar and Lyman-α forest power spectra to constrain the biases, we find that for DESI the expected 1−σ constraint is err[fNL]∼60. Ability for DESI to measure fNL through this channel is limited primarily by the aliasing and instrumental noise of the Lyman-α forest flux power spectrum. The combination of response approach and separate universe simulations provides a novel technique to explore the constraints from the squeezed-limit bispectrum between different observables.